A variety of lens systems have been designed to produce variable crossed cylinder powers for measuring or correcting astigmatic errors through the use of refractors. One such lens structure, the Snellen-Stokes system, uses two counter-rotating cylinder lenses of equal power and opposite sign to produce variable crossed cylinder powers. A second system, disclosed by Humphrey in U.S. Pat. No. 3,822,932, utilizes two pairs of Snellen-Stokes lenses, with the combined lens axis of one pair offset 45.degree. from the combined lens axis of the other pair. The pairs are alternatively counter-rotated to produce a desired combined cylinder power at a desired angle. Finally, a novel system described in my patent application Ser. No. 07/310,334 teaches use of four rotatable cylinder lenses of equal power and sign and a stationary lens of double the power and opposite sign of each of the cylinder lenses, to produce variable crossed-cylinder powers at various angles.
Diplopic errors typically are measured using two pairs of Risley prisms, with one pair aligned with each eye. Each Risley prism pair consists of two equal power prism lenses and is counter-rotated until fusion is obtained. The amount of rotation for one of the pairs of Risley prisms is related to the amount of base up/down defect present in the patient's eyes, while the rotation of the other pair corresponds to the base in/out defect present.
Each of the systems described above produces various optical powers through counter-rotation of pairs of lenses. Achieving counter-rotation, typically desired at equal rates in opposite directions, requires some type of timing mechanism such as a synchronized gear set. Consequently, no existing system is capable of generating crossed-cylinders throughout the 180.degree. field without using synchronized rotation of two or more lenses. Moreover, no existing variable prism system can be used to correct diplopia without resort to counter-rotation of multiple pairs of prism lenses.
Conventional trial frames, used to measure and correct a variety of refractive errors, similarly lack means for rotating two cylinder lenses independently. U.S. Pat. No. 1,337,265, for example, discloses trial frames having a pair of lens carriers alignable with a patient's eyes. Each lens carrier includes three open pockets in which up to one sphere, cylinder and prism lens from a battery of lenses may be placed. In accordance with conventional refracting techniques, however, only the pocket containing the single cylinder lens may be rotated. Although such trial frames do not compel synchronized rotation of two or more lenses, they require use of batteries of lenses and are incapable of generating variable crossed-cylinder powers throughout the 0.degree.180.degree. visual field.
U.S. Pat. Nos. 2,256,491 and 2,333,738 disclose alternative trial frames designs. Again, however, these trial frames include only a single rotation mechanism for each lens carrier, precluding independent rotation of more than one cylinder lens associated with each eye.